Voltage sags caused by either utility or customer side events cause industrial plant process control equipment to malfunction. Such voltage sags are classified as the momentary reduction in voltage for a period of 0.5 to 30 electrical cycles. These sags often accompany weather related events such as lightning, wind, and ice, that impact utility power circuits. They can also occur within the plant from electrical equipment failure. As protective devices clear these faults, voltage on adjacent circuits return to normal but not soon enough for nearby equipment to be unaffected. Control and logic circuits sense the voltage disturbances and often stop the flow of production. One type of industrial process, for example might be a sheet glass production plant. Modern industrial equipment which uses expensive electronic equipment is especially sensitive to voltage disturbances.
It is, of course, desirable that the process equipment "ride-through" these voltage disturbances. Process components may be the-modified to improve ride-through and desensing modifications such as constant voltage transformers can be installed where necessary. In any case, an effective voltage sag generator is necessary to initiate voltage sags in order to determine equivalent sensitivity to voltage disturbances and to bench mark desensing modifications as described above.
One type of sag generator used in an industrial context is described in the IEEE Transactions on Power Delivery, Vol. 11, No. 1, January 1996, Pages 526-532, entitled "A Three-Phase Sag Generator for Testing Industrial Equipment". Although the sag generator was mobile, it consisted of a diesel powered synchronous generator with about 15 kilowatts capacity. The use of a diesel type engine severely limits the use of this device because of pollution and the power rating is an order of magnitude less than desired for modern industrial applications. Solid state sag generators with variacs have been used but only for small loads.